Two hours before sunset on Wednesday, May 11, volunteers from The Coonamessett River Trust gathered at the lower dike to tag herring as part of a project to learn where they travel in the river system.

The idea is that data from the project can help guide the town as it restores the river and offer insight on why herring numbers have dropped in recent years.

“This study understands where they go in the Coonamessett River system and where the hang-ups are for migration,” said Linda A. Deegan, a fish biologist at The Ecosystems Center of the Marine Biological Laboratory, who is leading the volunteer-run project.

The study began last year when volunteers tagged 500 fish out of the run, estimated at 50,000 to 70,000, a tenth of what it could be, Dr. Deegan said. So far 13 fish have returned this year, offering insight on their life in the river system.

“We have learned a lot,” said Charles B. Cooper. “Some go to both ponds. Some visit one pond on the way up, stay three weeks and then visit another on the way down.”

Data also show river impediments. The fish struggle through the middle dike because the culvert is high in relation to the water level, creating a small waterfall.

“They tend to run up and stop, run up again and stop and fade back,” Dr. Deegan said.

This week the clock was ticking not only in terms of daylight but for tagging the most fish at the peak of their run, which is mid-May.

The crew, which included Christopher Neill, Charles B. Cooper, and Wendi B. Buesseler, had not tagged for several days because the herring stopped running last week, perhaps due to the rainy weather.

That night, however, Mr. Cooper estimated that there were at least 200 fish gathered in a deeper pool, which offered more protection against the gulls and osprey that flew overhead searching for a meal, compared to shallow waters.

“I feel them in there,” said Mr. Cooper, standing waist deep in the water and holding a net to corral the herring together.

Dr. Deegan quickly started picking fish out of the net to measure, and then insert a tracking device, a passive inductor transponder the size of a grain of rice, in each fish through a small incision and then seal it up with superglue. Clove oil in a holding bin sedated the waiting fish.

The device does not require a battery, so it lasts the life of the fish. Antennas, which are battery powered, are posted along the river at certain sites, and record the movement of the fish. The technology, about 10 years old, is the same used by EZ Pass to track vehicles.

Although herring have a long history in the area as a sign of spring fisheries, less is known about their journey between the ocean and freshwater rivers and pond where they spawn each spring.

“It’s not that herring have not been studied, but they have not been looked at from this perspective,” Dr. Deegan said.

Older records offer glimpses of what the herring runs were like hundreds of years ago. Evidence suggests that in the 1870s and 1880s one million fish were harvested from certain runs.

“Going back, the population now is just a shade of what was present before,” Bradford Chase, senior marine fisheries biologist with the Massachusetts Division of Marine Fisheries, said.

The population dropped significantly in the 1980s, with another alarming downturn in 2002, resulting in a harvest ban enacted in 2006 which is ongoing.

Between 2011 and 2015 numbers have slowly increased.

“The harvest ban was positive and specific restoration projects were positive, but it would be speculative to say the cause of this improvement. We need a little more time,” Mr. Chase said.

Massachusetts has 80 runs, 25 of which are being monitored to gather data on population sizes. Most have 50,000 to 100,000 fish while only a couple exceed 500,000 fish.

Scientists continue to investigate what is impacting herring numbers but point to a range of factors, such as dams that block river passageways, nitrogen-rich waters with abundant algae that crowd spawning areas, ocean acidification, climate change and habitat degradation. The herring also falls victim to ocean fisheries as bycatch.

“We refer to it as death by 1,000 cuts,” Dr. Deegan said. “Dams are a deep wound, stopping them cold; other effects are additive.”

Each spring, the majority of the alewife herring arrive first in mid-April followed about a month later by the blueback herring. This week at Coonamessett River they were tagging both types, measuring them and identifying their sex if possible. Adults stay in freshwater to spawn on average six weeks. Young herring stay longer traveling to the ocean in the early fall.

“We think they come back to pretty much the same river system,” Dr. Deegan said.

But unlike salmon, which return to the same river and within meters of where they spawned the year before, herring are less specific, Dr. Deegan said.

By tracking the herring closely in the Coonamessett River, the group hopes to ensure that the fish continue to return each year with growing numbers. Information learned at this local level may apply to other areas as well, Mr. Chase said.

The community has also become involved. Funds for the projects have been collected from an adopt-a-tagged fish program. People can follow their fish from the Coonamessett River Trust’s Facebook page. The Falmouth Rod and Gun Club also contributed funds to the project.

“It’s a hopeful migration, a sign of spring,” Mr. Chase said. “It’s important to have them move up the river every spring.”

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